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Weather & Climate Weather & Climate July 2018 “Weather is what you get; Climate is what you expect.” Weather consists of the short-term (minutes to days) variations in the atmosphere. Weather is expressed in terms of temperature, humidity, precipitation, cloudiness, visibility and wind. Climate is the slowly varying aspect of the atmosphere-hydrosphere-land surface system. It is typically characterized in terms of averages of specific states of the atmosphere, ocean, and land, including variables such as temperature (land, ocean, and atmosphere), salinity (oceans), soil moisture (land), wind speed and direction (atmosphere), and current strength and direction (oceans). Example of Weather vs. Climate The actual observed temperatures on any given day are considered weather, whereas long-term averages based on observed temperatures are considered climate. For example, climate averages provide estimates of the maximum and minimum temperatures typical of a given location primarily based on analysis of historical data. Consider the evolution of daily average temperature near Washington DC (40N, 77.5W). The black line is the climatological average for the period 1979-1995. The actual daily temperatures (weather) for 1 January to 31 December 2009 are superposed, with red indicating warmer-than-average and blue indicating cooler-than-average conditions. Departures from the average are generally largest during winter and smallest during summer at this location. Weather Forecasts and Climate Predictions / Projections Weather forecasts are assessments of the future state of the atmosphere with respect to conditions such as precipitation, clouds, temperature, humidity and winds. Climate predictions are usually expressed in probabilistic terms (e.g. probability of warmer or wetter than average conditions) for periods such as weeks, months or seasons. A prediction is a probabilistic statement of something that could happen in the future based only on what is known today. Climate projections are long-range predictions of the future climate based on changing atmospheric conditions, such as increased or decreased pollutants due to emissions from the burning of fossil fuels (coal, oil, gas). Example of Climate Prediction: Three - Month Outlooks Temperature Precipitation Climate predictions and projections never specify the occurrence of weather The maps above show temperature (precipitation) probabilities for above (A) and below (B) normal (median) conditions. In regions where no strong climate signals exist, a forecast of equal chances (EC) for either A, B, or near-normal conditions is made. events. Weather-Climate Example of Climate Projection Links Atmosphere-Ocean General Circulation Model Projections of Surface Warming 2020 - 2029 2090 - 2099 There are many ways in which climate and weather are connected. For example: The frequency and intensity of tropical cyclones are influenced by changes in sea-surface temperature and upper- level wind patterns in the tropics that occur on monthly, seasonal, decadal, and longer timescales. The maps above show projected surface temperature changes (°C) for the early and late 21st century relative to the period 1980-1999. This is the average projection for the A2 scenar- io averaged over decades 2020-2029 (left) and 2090-2099 (right). The A2 scenario is based on the assumption of a very heterogeneous world with high popula- tion growth, slow economic development and slow technological change. Mid-latitude storms are steered by the jet Shifts in the position of the jet streams in stream and get their mid-latitudes can affect where storms will energy from temperature develop. These jet-stream shifts can occur contrasts between on seasonal time scales (in association with different air masses. El Niño and La Niña) and also on weekly Relatively slow times scales, associated with planetary variations in the global- scale meanders of the jet-stream that are scale temperature sometimes related to changing conditions distribution are the basis in the tropical atmosphere on sub-seasonal for making outlooks on time scales from time scales. one month to several seasons. References Weather.gov NOAA/Climate Prediction Center NOAA/Earth System Research Laboratory Wind in the upper troposphere (200-hPa = 11.3 AMS Glossary of Meteorology km) averaged over the months of January, February and March. Thick black arrows show IPCC Report 2007 the locations of jet streams during the El Nino of 1999 and the La Nina of 1989..
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